In recent years, various proposals for further improvement of the light utilization efficiency and the energy saving in optical elements such as displays, light-emitting diodes, and solar cells have been made. For example, it has been known in a liquid crystal display to raise an aperture ratio of the display device by forming a transparent planarization film by application on a TFT element and then forming pixel electrodes on the planarization film (see the patent document 1 below). Further, in an organic EL device, it was also proposed to raise an aperture ratio thereof like a liquid crystal display by adopting a method for extracting light emitted by a luminous layer, which exists on a planarization layer formed on a TFT element by application, from the opposite side of the TFT element (a top emission method) instead of a method for extracting light emitted by a luminous layer, which is formed by vapor deposition on a transparent pixel electrode formed on a substrate, from the substrate side (a bottom emission method) (see the patent document 2 below).
Furthermore, delay in signal on wiring becomes a problem accompanying to high resolution, increasing size, high image quality, and 3D indication of a display. An incoming signal to a TFT becomes shorter by increasing of writing speed of an image data (frame frequency) but extension of the wiring width for lowering a wiring resistance is limited from a demand of high resolution. Because of this, it was proposed to solve the problem of signal delay by increasing the thickness of the wiring (see the non-patent document 1 below).
As materials of the planarization film for the TFT substrate, there has been known a material which comprises an acrylic resin and a quinonediazide compound (see the patent documents 3 and 4 below). Though properties of the material do not deteriorate sharply at a high temperature of 200° C. or more, decomposition thereof begins gradually at a high temperature of 230° C. or more and there occur problems such as reduction of a film thickness and lowering of transparency due to coloring of a transparent layer by a high temperature processing of the substrate. This material, therefore, cannot be used particularly in a process for forming a film on a transparent layer at a high temperature with devices such as a PE-CVD. Further, the acrylic material is not a best material for use in a high temperature process or for use to a device influenced by impurities, as decomposition products thereof affect badly to the luminous efficiency and the life of an organic EL element. Furthermore, the acrylic material to which a heat resistance is given has simultaneously a high dielectric constant generally. The material, therefore, has such problems that electricity consumption becomes larger due to the increase in the parasitic capacitance by an insulation film and the image quality decreases due to the delay of a driving signal in a liquid crystal element. Though the capacitance of an insulating material with a large dielectric constant can be made small, for example, by making a film thickness large, it is not preferable to make the film thickness large as it is difficult to form a coating film of e.g. 5 μm or more in thickness with a uniform film thickness on a large glass substrate by a slit coating method etc. generally and the amount of the material used becomes large.
The polysiloxane, in particular, silsesquioxanes is known as a material with a high thermal resistance and a high transparency. Though the silsesquioxane is a polymer constructed by a three-functional siloxane structure unit: RSi(O1.5) and is in between an inorganic silica (SiO2) and an organic silicone (R2SiO) with respect to the chemical structure, it is a singular compound as it is soluble in an organic solvent but the cured material thereof shows a high thermal resistance which is a characteristic of inorganic silica. Furthermore, as the polymer having a siloxane skeleton has a low dielectric constant compared to organic polymers generally, it is expected as a material for a transparent insulation film with a low dielectric constant. When the polysiloxane is used as a component of a photosensitive composition, it is necessary that the polysiloxane is soluble in a developer such as a tetramethylammonium hydroxide aqueous solution. Therefore, there was proposed a photosensitive composition comprising an acrylic copolymer which is formed by copolymerizing a silsesquioxane compound, in which an acrylic group is given to a particular cage-type silsesquioxane, with an unsaturated carboxylic acid, an unsaturated compound containing an epoxy group, and an olefinic unsaturated compound, and a quinonediazide compound (see the patent document 5 below). However, the composition comprising a complicated system described above gives a cured material with an insufficient thermal resistance due to the thermal deterioration of organic compounds except for the polysiloxane, as the amount of the organic compounds is large. In addition, the problems of coloration and gas generation due to the decomposition cannot be ignored.
As a photosensitive composition comprising the polysiloxane and the quinonediazide, there was proposed a photosensitive composition comprising, for example, a combination system of a polysiloxane insoluble in a developer and a polysiloxane soluble in a developer and a quinonediazide compound, by which a ‘pattern’ sagging, that is, the deterioration of the resolution caused by flowing of developed hole or line patterns at a heat curing, was prevented (see the patent document 6 below). However, when a polysiloxane insoluble in a developing solution is used, it will cause generation of a development pattern defect due to undissolved residues after development or re-adhesion of insoluble materials eluted into the developing solution during the development.
As methods for retaining solubility of the polysiloxane in a developing solution by a group except a silanol group, there were proposed a method of acylating a part of phenyl groups in phenylpolysiloxane (see the patent document 7 below) and a method of using a cage-type silsesquioxane compound having a quinonediazide structure (see the patent document 8 below). As these siloxanes have stable developer-soluble groups, even if a silanol group reacted during development, problems of generating an undissolved layer and undissolved residues are reduced. However, as the cured materials of these polysiloxanes have poor resistance for chemicals such as a stripper for photoresist, usable uses will be limited.
By the way, when a photosensitive siloxane composition containing a diazonaphthoquinone derivative as a dissolution inhibitor is developed using a 2.38 wt-% tetramethylammonium hydroxide (hereinafter, refer to “TMAH”) aqueous solution as a developer, it is possible to forma practical positive pattern after exposure and development if the rate of dissolution in a 2.38 wt-% TMAH aqueous solution of the polysiloxane is 100 Å/second or more. However, a polysiloxane having a silanol group and soluble in a TMAH aqueous solution is a polymer having a relatively low molecular weight and when this polysiloxane is used, the ‘pattern’ sagging is generally caused in a heat curing process.
The polysiloxane becomes a high molecular weight by the dehydration condensation of silanol groups by heat. This reaction proceeds faster as the temperature is higher. However, it is difficult to control the ‘pattern’ sagging at a high temperature, as the polysiloxane becomes temporarily a low viscosity at a high temperature.
The ‘pattern’ sagging can be reduced by increasing a molecular weight of a polysiloxane. However, if the molecular weight of a polysiloxane is increased, the polysiloxane becomes slightly soluble in a 2.38 wt-% TMAH aqueous solution as described in the patent document 6. Therefore, it will cause problems such as deterioration in resolution, low sensitivity, and pattern defects after development. As another method for solving the ‘pattern’ sagging, there was proposed a method for reducing the ‘pattern’ sagging despite a low molecular weight material by synthesizing a polysiloxane, which is a polysiloxane having a low dissolution rate in at least two kinds of polysiloxanes having different dissolution rates, in the existence of a basic catalyst (see, the Patent document 9 below). In this method, the rate of dissolution in a 2.38 wt-% TMAH aqueous solution of the polysiloxane mixture is adjusted by changing a mixing ratio of polysiloxanes having different dissolution rates and a positive-type photosensitive composition having a high sensitivity can be obtained.
Generally, in the subsequent development process after the light irradiation process, a relatively long time development from 30 seconds to 120 seconds are performed for controlling a pattern size after development. It is, therefore, necessary to make the dissolution rate slower as the thickness of the film becomes thinner. However, in the method described in the patent document 9, it is necessary to increase an amount of a slightly soluble polysiloxane for delaying the dissolution rate when the thickness of the film is thin or the photosensitive composition is applied to a process in which a long time development is performed. Because of this, there is a problem such that development residues called scum remains.